Electro-fusion fittings

ABSTRACT

An electro-fusion pipe coupler includes an outer shell and an electric resistance heating coil secured therein by an intervening layer of injection moulded thermoplastic material. The coupler is made by winding the wire onto a core which consists of two separable parts, inserting the core into the tubular shell, attaching contacts to the wire ends and inserting them into holes provided in the shell, and injecting molten material into the space defined between the core and the shell to form the layer. For injecting the material the core parts define radial runners therebetween connected to a central sprue inlet and opening at respective outlets spaced around the core.

BACKGROUND OF THE INVENTION

This invention relates to electro-fusion fittings and their manufacture.Electro-fusion fittings are used for making couplings in thermoplastics,most commonly polyethylene pipe. The invention is particularly concernedwith fittings intended for connecting two pipe lengths end-to-end, suchfittings being frequently referred to as welding muffs or sleeves.Conventional welding sleeves consist of a hollow cylindrical body ofthermoplastic material with a coil of electrical resistance heating wireat its inner surface. In use, the pipe ends are pushed into the oppositeends of the sleeve and an electric current is supplied to the heatingcoil so that the material of the pipes and fitting body is melted andfuses together to form a secure, leakproof coupling between the pipeends.

Different methods can be employed for manufacturing electro-fusionsleeves. Most convenient in the case of sleeves of relatively smalldiameter, and hence having bodies of relatively small mass, is atechnique wherein the body is produced by a normal injection mouldingprocess. The heating wire, coated in the thermoplastic material, may bewound onto an injection mould core and then have the body injectionmoulded in situ over the wire coil.

This technique is not ideal for manufacturing large diameter fittings,however, because tooling costs are very high and the production rate isvery low due to a long cycle time (in the order of 20 minutes) whicharises through the time it takes to fill the mould cavity with moltenplastic and the subsequent curing time necessary before the mould can beopened. As a solution to the problem of producing economically largediameter fittings, it has been proposed in GB-A-2036518 to form the bodyby cutting a length off from a pipe of thermoplastic material. Theheating coil is applied to the interior of the pipe length by a processwhich involves: expanding the pipe length and introducing into it amandrel onto which the wire, either bare or coated with thermoplasticmaterial, has been wound; the pipe length is shrunk onto the mandrel andwinding in a furnace; electric current is supplied to the heating wireto cause the wire turns to expand while the mandrel is expanded orshrinking of the pipe length is continued, and the thermoplasticmaterial is caused to flow between the turns; and, after cooling, thewire ends are connected to contact pins previously mounted on the pipelength. Thus, the procedure is complicated and there still exists a needfor a reliable and economic manufacturing method.

SUMMARY OF THE INVENTION

In accordance with one aspect the present invention provides anelectro-fusion pipe coupler comprising an outer shell in the form of aselfsupporting cylinder, an electrical resistance heating windingcomprising a coil of wire coated with thermoplastic material housedwithin the shell, and a layer of thermoplastic material moulded in situbetween the winding and shell and securing the winding within the shell.

In such a construction, the main purpose of the outer shell is to impartthe necessary strength to the fitting. While it is preferred that theshell is made from thermoplastic material, it may be possible for it tobe made from other materials such as metal. This possibility existsbecause it may not be necessary for the shell to partake in the pipewelding process as the moulded thermoplastic layer, in combination withthe wire coating material, may be adequate to ensure a leakproof jointbetween two pipe ends.

During the injection moulding step the shell constitutes an outer mouldpart confining the mould cavity with a core onto which the wire iswound. Consequently expensive moulding apparatus can be obviated. Evenif the shell is made of thermoplastic material, an integral bond unitingthe shell and injection moulded layer will not necessarily be obtainedby the injection moulding process, although the two parts will be indirect face-to-face contact. In order to prevent the moulded layerbreaking away or delaminating from the inner surface of the shell due tothe shrinkage which tends to occur naturally during curing of themoulded material, it is preferable for the moulded layer to be formed ateach end with an external projection in abutment with an axiallyoutwardly facing shoulder defined by the shell. The shoulder could bedefined by the end face of the shell, or by a rebate at the inner edgeof the end face. Also, the shoulder could be inclined to the axis, orformed in some other way to provide a secure interlock with the mouldedlayer. However, very satisfactory results have been obtained with aradial shoulder defined by an annular groove machined in the innersurface of the shell at a small distance in from the end.

It is preferable for the moulded layer to extend axially from end to endof the shell. If the shell is made of thermoplastic material, however,two discrete layers could be possible, these layers being associatedwith winding portions arranged for cooperation with respective pipe endsduring the welding procedure.

At each end of the coupler the lining formed within the shell by themoulded layer can conveniently be chamfered to provide a lead-in for theends of the pipes to be coupled together.

A further advantage of the invention is that by limiting the amount ofinjected material, dimensional accuracy of the finished bore is moreeasily achieved.

According to another preferred feature, the coupler comprises a contactpin assembly extending through the wall of the shell and comprising aninner part held in place by the moulded layer, and an outer partconnected to the inner part and providing a terminal portion. The innerand outer parts are joined releasably, e.g. by a threaded connection.This contact construction has certain advantages which will become clearfrom the description that follows.

In accordance with a second aspect the invention provides a method ofmaking an electrofusion sleeve coupling comprising the steps ofpreparing a cylindrical shell, winding a coil of electrical resistanceheating wire coated with thermoplastic material onto a mould core,inserting the core and winding thereon into the shell with a radialspace defined therebetween, injecting thermoplastic material to fill thespace, and removing the core so that the winding is retained in theshell by the moulded material.

The preparation of the shell preferably comprises machining the shelland thereby forming an axially outwardly directed shoulder adjacent eachend thereof so that the layer of material subsequently moulded in situinside the shell abuts against the shoulder for the purpose which hasbeen explained above.

Conveniently, after inserting the core into the shell, a contact partcan be attached to the wire end and introduced into a radial holeprovided in the shell so that the contact part will be secured in placeby the moulded material. During the moulding step the contact part canbe secured in position by a fixing device engaged with the contact partfrom outside the shell. With the connections between the contact partsand wire ends embedded in the moulded material, they are protected anddo not form protrusions against which the pipe ends may snag when theyare inserted into the coupler.

The wire is preferably wound in two tightly wound sections in each ofwhich the adjacent turns contact each other, with a link portioninterconnecting these two sections.

The present invention is aimed mainly at the manufacture of largediameter electro-fusion fittings, but there is no reason why it shouldnot be applied also to the production of smaller diameter fittings ifdesired.

Most conveniently the shell is prepared from a tubular blank, e.g. sawnoff from an extruded thermoplastic pipe, or fabricated from an extrudedribbon as described in more detail hereinafter. The blank material ispreferably the same as the injection moulded material, but this is notessential. The inner surface of the blank is machined to a uniformdiameter but is left with a rough surface finish to provide a key toimprove the adhesion of the moulded layer to the shell.

In accordance with a further aspect of the invention there is provided amould core for use in manufacturing on electro-fusion sleeve, comprisingtwo circular core parts, means for securing the core parts together todefine an axially continuous cylindrical surface for receiving a windingof electric resistance heating wire, an inlet for molten plasticmaterial in an outer end face of one core part, and an outlet for themolten material defined on the cylindrical surface at the interfacebetween the core parts.

In a preferred core construction the inlet extends axially through theone core part and communicates with a plurality of radial runnersdefined between the two core parts and opening at respective outletsspaced apart around the cylindrical surface.

The outer surfaces of the core parts may be provided with means, e.g.,circumferential rims or projections for holding the winding in placebefore and during the injection moulding process.

A clear understanding of the invention in its various aspects will behad from the following more detailed description given with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial cross-section through an electro-fusion sleevefitting embodying the present invention;

FIG. 2 is an enlarged detail showing that part of the fitting circled inFIG. 1;

FIG. 3 is an end view of the fitting shown in FIG. 1;

FIG. 4 is a schematic longitudinal crosssection showing an apparatus forpreparing a shell blank for manufacturing the fitting of FIG. 1;

FIG. 5 is a top plan of the apparatus shown in FIG. 4;

FIG. 6 is a side elevation, shown partially in section taken along theline VI--VI in FIG. 7, of a mould core for use in manufacturing thefitting shown in FIG. 1;

FIG. 7 is a right hand end elevation of the core shown in FIG. 6;

FIG. 8 is a section through the core taken along the line VII--VII inFIG. 6; and

FIG. 9 is a transverse section illustrating an electrical contactassembly prepared ready for the injection moulding step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIGS. 1 to 3, there is illustrated a pipe couplerin the form of an electrofusion sleeve fitting. The body of the fittingconsists mainly of a cylindrical shell or carcase 1 on the interior ofwhich is arranged an electrical resistance wire winding 2 held in placeby an intervening layer 3 of injection moulded thermoplastic material.The layer 3 extends from end-to-end of the shell and at its ends isformed with chamfered faces 4 defining a lead-in for each pipe end tofacilitate insertion of the pipe ends to be joined into the sleeve. Atthe medial plane the layer 3 defines a plurality of circumferentialspaced studs which project inwardly and define stops 5 for the pipe endsto abut against, thereby to ensure correct positioning within the sleevein preparation for the welding process. At a small distance from eachend an annular groove 6 of rectangular cross-section is machined in theshell 1 and the material of the layer 3 fills this groove to provide amechanical interlock between the shell and moulded layer. This interlockprevents the layer 3 breaking away from the shell as a consequence ofthe radial and axial shrinkage effects which occur in the layer 3 whileit is cooling as part of the injection moulding process.

The wire winding is wound in two coil sections which are contiguous dueto an integral wire section linking them and depicted schematically inbroken line in FIG. 1. The coil sections are arranged for cooperationwith respective pipe ends in well known manner. The winding is formedfrom wire coated in thermoplastic material which is preferably the samematerial as that of the layer 3 and ideally also the same as thematerial of the shell 1. In each of the two coil sections the turns ofthe winding are arranged in a single layer and in abutment with eachother. By virtue of being injection moulded in situ, the layer 3 adheresstrongly to the shell and the coating of the wire winding, but it is notunited therewith, that is, until the welding process when the heatgenerated in the winding by the electric current passed through itcauses the thermoplastic material at the boundary faces to melt and fusetogether, at least in the regions of the two coil sections, whereby astrong fusion bond is obtained between the outer surface of the pipe endin the sleeve, and the material of the wire coating, layer 3 and theshell 1.

The ends of the wire are connected to contact pin assemblies forconnecting the winding to a source of electrical energy during thewelding process. As may be best seen in FIG. 2, an eye connector 8 isattached to the wire end by crimping and an inner contact part 9 havingan enlarged inner end extends through the eye and into a radial holeprovided in the shell 1. The inner end of contact part 9 and theconnector 8 are embedded in the layer 3 so that they do not form anyobstruction to and will not be damaged by a pipe end being introducedinto the welding sleeve. At its outer end the contact part 9 has athreaded connection, in particular a tapped core into which an outercontact part 10 is screwed. The contact part 10 comprises a pin ontowhich a plastic shroud 11 is fitted or moulded to surround an outer pincontact end, and the inner end of the pin is screw threaded forengagement with the inner contact part, as shown. During the pipewelding process the two pin contacts of the fitting are connected to anelectric current source in well known manner.

Because the space between the shell 1 and the electrical winding 2 isfilled by the layer 3, it is not necessary for the shell to bemanufactured to close tolerances or to be expanded and/or shrunk whileapplying the winding into the shell. The shell could be prepared from atubular blank cut from an extruded plastics pipe, but an alternativemethod of making a tubular blank is shown in FIGS. 4 and 5. Aconventional plastics extruder 12 and an extrusion die 13 are used toproduce a flat ribbon extrudate 15 of essentially rectangular section.The extrudate is led to a winding drum 14 and is wound onto the drumuntil the desired thickness has been built up. A pressure roller 15 isused to press the extrudate onto the underlying layers already on thedrum to ensure close intimate contact. The cylindrical shell blank thusproduced may be subject to annealing and is then machined ready forproducing an electro-fusion fitting. The blank is machined over itsinner and outer surfaces, including the machining of the grooves 6, andis drilled with the radial holes to accept the contact terminalassemblies. As already mentioned above, the shell can be made to largetolerances and the machining needs only to give the approximatedimensions. It is actually best for the inside surface to be machined toa relatively rough finish, e.g. to provide a helical ridge whichincreases the surface area, to improve the adhesion of the injectionmoulded layer to this surface.

In the injection moulding step, i.e., during the formation of the layer3, the shell 1 constitutes an outer mould part delimiting the mouldcavity with a mould core. The core is shown in FIGS. 6 to 8, but itshould be noted that FIG. 6 is not a true section insofar as some parts,mainly bolt 24 and dowels 35, are shown out of true position for ease ofillustration. The core has an outer cylindrical surface withcircumferential end rims 20 for producing the chamfer faces 4 asdescribed above. Spaced a short distance in from the rims are furthercircumferential projections 21 which define axially inwardly directedradial stop shoulders which serve to locate the outer ends of therespective heating coil sections and to hold them in position duringinjection of the moulding material. The core is made in two axial halves22, 23 which are held together by a detachable bolt 24. To ensurecorrect coaxial alignment between the core halves, the core half 23 isequipped with two diametrically opposed location dowels 25 which entercorresponding bores 26 in the core half 22 when the two halves arebrought together. The core half 22 is equipped with an axial sprue bush27 defining an inlet channel 28 for the moulding material, the inner endof the channel opening into six radial runners 30 uniformly distributedaround the axis and having gates 31 opening at the cylindrical surfaceof the core. The runners are confined between the confronting faces ofthe core halves and have their gates shaped for producing the stops 5mentioned above. Accommodated in recesses in the outer end faces of thecore halves are extraction plates 32 fixed in position by bolts 33. Ineach plate 32 there are three key-hole shaped slots 34 which areundercut behind their narrow portions enabling enlarged heads of anextraction tool to be inserted and by a small angular rotation besecurely locked to the extraction plate and hence the core half. Withextraction tools so engaged with the two core halves they can be pulledapart and in opposite directions out from a completed electrofusionfitting, the bolt 24 having been removed first of course. It will benoted that the head of bolt 24 is located in a slot 35 in the extractionplate of core half 22. The corresponding extraction tool has aprojection arranged to enter this slot and if the bolt 24 has not beenremoved the extraction tool cannot be engaged with the extraction plate.For support and correct positioning of the core within the injectionmoulding machine, the extraction plates are provided with taperedlocation recesses 36. It will be noted that the sprue bush is fixed inthe core half 22 by its extraction plate 32.

The core halves are formed with removable core segments 38. When thecore is inserted into the shell these segments, or at least the spacesthey normally occupy, are positioned opposite the holes provided in theshell to receive the contact terminal assemblies, for reasons which areexplained below.

With the core assembled as shown in the drawings, the coated electricalresistance wire is wound onto the core to form the two coil sections,respectively abutting with their outer ends the stop faces of rims 21,with the wire ends being left protruding at the position of theremovable core segments 38. The core and winding are inserted axiallyinto the previously prepared shell. The segments 38 are removed so thatthe eye connectors 8 can be crimped onto the wire ends, and the innercontact parts 9 are inserted through the eyes and into the holes in theshell. To secure them firmly during the injection moulding process, amould tool bung 39 (FIG. 9) generally formed as a bolt is screwed intoeach contact part 9 and serves to clamp it firmly against the interiorsurface of the shell. The segments are then replaced and the entireassembly of shell and core is loaded into a suitable injection mouldingmachine. The thermoplastics material is injected through the sprue bushand enters the cavity confined between the shell 1 and core via therunners 30 and gates 31 to fill this space and thereby form the layer 3for holding the winding on the interior surface of the shell. After anappropriate curing time, the assembly is removed from the mouldingmachine and the core halves 22, 23 can be removed by extraction toolsengaged with the core halves as explained above. All that remains tocomplete the fitting is to remove the mould tool bungs 39 and insert intheir place the outer contact parts 10. If preferred, to remove any riskof damage during storage and/or transportation, the outer contact partsmay be fitted only when the electro-fusion is ready to be used toproduce a welded pipe joint. It will be noted that apart from beingconvenient from a manufacturing viewpoint the two-part contact assemblyhas the advantage that different outer contact parts could be applied,e.g., to suit the particular contact configurations of different controldevices which may be used to supply electric power to the fitting.

It will be understood that modifications are possible without departingfrom the scope of the invention. Hereby as an example it is mentionedthat other forms of connector may be used to connect the wire ends tothe contacts, rather than the eye connectors shown and described, or ifthe contacts are suitably made a direct connection with the wire ispossible. Thus the wire end could be inserted into a transverse boreprovided in the inner end of contact part 9 and be securely clampedtherein by a grub screw screwed into the tapped core of part 9 ahead ofthe outer contact part 10.

I claim:
 1. An electro-fusion pipe coupler for connecting end-to-end twopipe lengths, the coupler comprising a cylindrical body and a boreextending through the body from one end to the other end thereof toreceive end portions of the two pipe lengths inserted into therespective ends of the body, the body comprising a prefabricated outershell having the form of a self-supporting cylinder of substantiallyconstant diameter, an electrical resistance heating winding housed inthe shell, said winding comprising a coil of wire coated withthermoplastic material, and a layer of thermoplastic material injectionmoulded in situ between the winding and said prefabricated outer shelland securing the winding within said prefabricated outer shell.
 2. Apipe coupler as claimed in claim 1, wherein the wire coating and saidlayer are made of the same thermoplastic material.
 3. A pipe coupler asclaimed in claim 1, wherein said outer shell is made of thermoplasticmaterial.
 4. A pipe coupler as claimed in claim 3, wherein said outershell and said layer are made of the same kind of thermoplasticmaterial.
 5. A pipe coupler as claimed in claim 4, wherein saidthermoplastic material is polyethylene.
 6. A pipe coupler as claimed inclaim 3, wherein said outer shell comprises a length of pipe cut from anextruded thermoplastic pipe.
 7. A pipe coupler as claimed in claim 3,wherein said outer shell is formed from several layers of extrudedthermoplastic ribbon wound continuously, layer upon layer.
 8. A pipecoupler as claimed in claim 1, wherein the moulded thermoplastic layerextends along substantially a full length of said outer shell.
 9. A pipecoupler as claimed in claim 8, wherein the thermoplastic layer is formedwith a chamfered face at each end thereof for guiding pipe ends to bejoined into the coupler.
 10. A pipe coupler as claimed in claim 8,wherein said outer shell has opposite ends and defines respectiveshoulders herein adjacent said opposite ends, the shoulders beingdirected generally axially away from the winding, and wherein saidmoulded thermoplastic layer includes external projections whichrespectively abut said shoulders.
 11. A pipe coupler as claimed in claim10, wherein each said shoulder is defined by a circumferential groove inan inner surface of said outer shell.
 12. A pipe coupler as claimed inclaim 1, including a hole in said outer shell and a contact assemblyextending through said hole and comprising an inner contact part towhich an end of the winding wire is secured and an outer contact partconnected to the inner part and providing a terminal portion.
 13. A pipecoupler as claimed in claim 10, wherein the inner contact part is heldin position by the moulded thermoplastic layer.
 14. A pipe coupler asclaimed in claim 10, wherein the inner and outer contact parts have areleasable connection with each other.
 15. A pipe coupler as claimed inclaim 1, wherein said layer of thermoplastic material is adhered to bothsaid winding and said prefabricated outer shell.